Fix visual/push notifications for deleted messages.

[drrosa]

As discussed in CZO thread and #26166 (comment), we should fix the following visual/push notification quirks for deleted messages:

• Deleting a message completely does not revoke mobile push notifications.

We should also update /help/delete-a-message.md#message-notifications once we've fixed the above.

The message editing part of this issue has been split out into #30316.

[drrosa]

FYI @prakhar1144.

[zulipbot]

Hello @zulip/server-message-view members, this issue was labeled with the "area: message-editing" label, so you may want to check it out!

[timabbott]

Tagging as an 8.0 priority in that this seems like significant incorrect behavior, and I think it might not be super hard to fix, in that we do various similar things correctly.

[prakhar1144]

I have started working on "Deleting a message completely does not revoke mobile push notifications."

[gnprice]

From the mobile client perspective: revoking notifications will work great on Android, only requiring the server to send an appropriate FCM blob in the same way it does when the message is read.

That functionality isn't currently available in our iOS app; it's tracked as zulip/zulip-mobile#3119 . But I believe that's basically orthogonal to this issue — the implementation of this issue should send those revocations the same way as we send other revocations, which is for Android only, and then in the future when we have an iOS client handling revocations and start sending them there, we'll do so uniformly for all causes of revocations, including the ones added as part of this issue.

Editing the content of the message in a notification is another matter. Properly supporting that will mean adding a new type of push-notification blob to the protocol between the server and the client. It would be good to do, but should be tracked as its own issue, because unlike the revocation cases it will be more work and will happen later. (Probably after 8.0 and after the Flutter migration of the mobile app.)